It is very common for batteries which are used in portable communication devices, such as two-way radios, to have a thermistor and a battery capacity resistor. The thermistor is used by a battery charger during the charging of the battery to determine the temperature of the battery and whether the battery is being charged properly. The capacity resistor is typically used by the charger to determine the capacity of the battery, prior to the battery being charged. The battery charger upon determining the battery capacity (e.g., 1000 milli-amp-hour maH) will select the proper charging rate to use, in order to optimally charge the battery.
Referring to FIG. 1, there is shown a prior art battery charging scheme consisting of a charger 102, radio battery 106 and radio 104. Radio 104 contains positive (B+) and negative (B-) battery terminals which are coupled to radio battery 106 via battery contacts 116 and 114, respectively. Battery 106 contains one or more battery cells 108, which dictate the voltage and current capacity of battery 106. Also included as part of the battery 106, are reverse discharge protection diode 118, a battery temperature indicator, such as thermistor (Rt) 112, and a battery capacity indicator, such as resistor (Rc) 110.
Charger 102 consists of a charger monitor circuit 128, which can consist of a well known microprocessor or microcontroller as known in the art and appropriate control software. Charger monitor circuit 128 controls charger control circuit 130 which provides current to battery 106 in order to charge the battery. A control signal is sent from charger monitor circuit 128 to charger control circuit 130 via bus 140, the control signal informs charger control circuit 130 as to how much current to source via line 129 to battery 106.
Charger monitor circuit 128 contains three analog-to-digital (A/D) ports 120, 122 and 124. A/D port 120 monitors the voltage on the B+ line. A/D port 122 senses the resistance of capacity resistor Rc 110, and A/D port 124 in turn senses the resistance of thermistor Rt 112 as its' resistance changes according to temperature which occurs once the battery begins charging. A/D ports 122 and 124 include external pull-up resistors 142, 144 respectively which are used to determine the resistance of Re 110 and Rt 112, by determining the voltage level at A/D ports 122 and 124, respectively.
Charger 102 and battery 106 in the prior art scheme use four lines connecting the charger 102 and battery 106. These lines include a B+ line 132 which provides the current to the battery, an Rc line 134 which is used to sense the capacity resistor 110, a thermistor sense line 136 which is used to sense the resistance value of thermistor 112, and a B- (ground) line 138.
Typically, charger 102 will continue charging the battery at a predetermined rate (1C) until the battery is charged to approximately 90% of its' full capacity. At this point, battery 106 reaches a predetermined temperature or temperature rise characteristic, as indicated by the thermistor sense line 136, and the charger 102 changes the charge rate to a lower charge rate, commonly referred to as a trickle charge rate. If the battery were to continue charging at the full charge rate (1C), it would become exothermic, and the battery cells could be damaged. The trickle charge rate allows the battery 106 to complete its' charge at a lower charge rate.
In the past, typical trickle charge rates have been in the approximate range of C/10. However, to improve battery life and battery performance, it is desirable to reduce the trickle charge rate, for example to a rate of C/20. Lowering the charge rate can present a problem for portable radio products whose standby currents are higher than the charger's trickle charge rate. If a radio, such as radio 104, is left "on" while the battery is charging at the lower charge rate, the battery 106 will never reach a fully charged state. This problem is further exacerbated if the radio 104 goes into a receive mode while the battery is being charged, drawing even higher current. For example, a battery that uses a charge rate of C=1000 mA and a trickle charge rate of 50 mA would never reach its' full charge capacity if the radio were left "on" while in the charger with a standby current of 90 mA. Even less capacity would be achieved if the radio were to scan through receive channels at, for example, a rate of 300 mA. Thus, a user could remove the radio from the charger and potentially be faced with a partially charged or even fully discharged battery.
Another problem arises if a radio with a fully charged battery is placed in the charger with the radio turned "off". The charger, recognizing the fully charged battery, goes into trickle charge and continues to trickle charge even to the point of overcharging the battery. In the past, for certain battery chemistries such as nickel cadmium, overcharging was not an issue because of the robustness of the chemistry. However, with the increasing interest in lithium ion and nickel metal hydride technology, the issue becomes more serious as these types of battery technologies can be easily damaged by overcharging.
Deep discharge of a battery is yet another related issue that can occur between the battery and the radio. A deep discharge condition exists when the battery voltage continues to drop below its minimum recommended cut off threshold. This phenomena can occur when a radio is left "on" and the battery continues to drain, even after the radio stops functioning, due to the load presented by the radio. Draining the battery down to these low voltage levels can lead to cell reversal and shorts which adversely affects the battery's cycle life performance.
Hence, there is a need for a battery and charger system capable of completing a charge cycle and maintaining a fully charged battery without overcharging or undercharging the battery. A battery that also prevents deep discharge would be a further enhancement to the system.